The principle objective of this proposal is to develop efficient new procedures for the stereoselective synthesis of a variety of biologically active molecules. A major focus is to be on new methods of ring formation, a basic reaction that is ubiquitous in natural products synthesis. New aspects of cationic, radical, and carbanionic cyclizations will be studied, with particular emphasis on the stereoselectivity of these processes. Findings will then be applied to the synthesis of alkaloids, steroids, and other biologically active molecules.